1. Field of the Invention
The present invention relates generally to a gravitational accelerometer for use in calculating an estimated vehicle speed of an automotive vehicle, and more particularly to a gravitational accelerometer provided with a zero adjuster or zero correction unit.
2. Description of Related Art
In a wheel behavior control apparatus for anti-locking, etc., estimation of the vehicle speed which is the basis for computing slippage or spin, and that of the coefficient of friction between tires and the road surface, are important factors on which the control performance is directly depends. The estimation of the coefficient of friction between the tires and the road surface is normally calculated from an estimated value of the vehicle acceleration reached during control, and the estimated value of the vehicle acceleration is computed based on the estimated value of the vehicle speed. Accordingly, the performance of the vehicle behavior control apparatus greatly depends on the accuracy of the estimated vehicle speed.
Incidentally, if the vehicle speed is estimated only from the wheel speed, the accuracy in the estimation is markedly deteriorated when an excessive slippage or spin takes place on the wheels.
Therefore, if the estimated vehicle speed and acceleration are obtained based on the information of both the acceleration detected by a reliable accelerometer and the wheel speed, the accuracy in the estimation can be remarkably improved. For the accelerometer, a gravitational accelerometer is generally employed.
However, as schematically shown in FIG. 15, although the acceleration sensor of the gravitational accelerometer is arranged to convert the amount of displacement of a weight or pendulum F arising from acceleration, into electrical signals by using resistors, piezoelectric elements, differential transformers, etc. for detection of acceleration, it is difficult to avoid influences due to fixing accuracy, and somewhat long-term variations such as electrical drifts and gain fluctuation, etc., as well as influences due to inclination or gradient of road surface in principle.
Accordingly, when the gravitational accelerometer is employed, it becomes important how to effect the zero point correction and sensitivity correction including correction of the road surface inclination.
With respect to the above problem, there has conventionally been proposed a zero adjuster or zero correction unit for the gravitational accelerometer, for example, in Japanese Laid-Open Patent Publication (unexamined) No. 4-223275.
The zero adjuster of the gravitational accelerometer as referred to above first finds a difference between the vehicle acceleration Aw estimated from the wheel speed and the acceleration Ac obtained by the gravitational accelerometer after the correction, and subsequently outputs a correction quantity Ao for this cycle by adding to or subtracting from a correction quantity obtained in a previous cycle, a value as obtained through multiplication of the above difference by a correcting speed. Using the correction quantity Ao, the acceleration obtained by the gravitational accelerometer for the present cycle or next cycle is to be corrected.
FIG. 17A shows the state in which an automotive vehicle provided with the conventional gravitational accelerometer as described above and travelling or running at a constant speed, starts braking in the course of a downhill, and stops on a generally horizontal road continued to the downhill. FIG. 17B shows variation of the wheel speed. FIG. 17C represents variations of the vehicle acceleration Aw estimated from the wheel speed, acceleration Am based on the gravitational accelerometer, and the variable correction quantity Ao obtained for each cycle. FIG. 17D denotes variation of the acceleration Ac obtained from the gravitational accelerometer after correction by using the correction quantity Ao.
In FIGS. 17A-17D, a region I shows the state in which the vehicle is travelling or running on a horizontal road at a constant speed, a region II the state in which the vehicle is running on a downhill at a constant speed, a region III the state in which the vehicle is running on the downhill while braking, and a region IV the state in which the vehicle runs and stops on a horizontal road while braking.
Here, when the zero adjuster of the conventional gravitational accelerometer as described above is used, in the case where the vehicle travelling on the downhill while braking again reaches the horizontal road, and runs on the horizontal road while braking as it is, i.e., in the region IV, the correction quantity Ao does not immediately fall to zero, but reaches zero after a considerable delay, as shown in FIG. 17C, in spite of the fact that the road is of the horizontal road in the region IV.
Therefore, as shown by an arrow Z in FIG. 17D, the acceleration Ac of the gravitational accelerometer corrected by using the correction quantity Ao is calculated and outputted to be undesirably small in the direction of speed reduction, and since control of a low friction coefficient (the friction coefficient is represented as .mu. hereinafter) is effected during this period, the braking force becomes insufficient, resulting in such an inconvenience as an increase of the stopping distance.
As described above, the zero adjuster of the conventional gravitational accelerometer follows the variation of the zero point comparatively quickly generally during running at a constant speed or in the absence of slippage or spin, but the follow-up becomes slow, for example, during braking on the way of a downhill, thus giving rise to such a problem as the increase of the stopping distance as referred to above.